In simple terms Error 404 means, that the information you're searching for could not be found on the server. It's a client-side Error, which means either that web-page has been moved or completely removed and the preferable change has not been done to that URL, or else it can even occur when someone type the wrong URL. In simple words that web page does not exist with the entered URL.

The HTTP 404, 404 Not Found and 404 (pronounced "four oh four") error message is a Hypertext Transfer Protocol (HTTP) standard response code, in computer network communications, to indicate that the client was able to communicate with a given server, but the server could not find what was requested.